Technology

The one thing to criticise about steam (and that they're slowly, but surely, losing a EU court case over) is inability for customers to sell their games.

Their marketshare is organic, based on being the choice of store both from the customer and developer POV. As a customer you get the usual painless returns, great interface, community features and whatnot, as a developer you get plenty of store features which make life and customer acquisition and gamedev a lot easier (things like playtesting, next fests) and most of all you get customers because steam has lots of customers and a real, I mean real good, recommendation algorithm. Sure, Epic wants a smaller cut but you're also not going to sell much, there, which is why they had to lure devs in with advances, guaranteed sales, etc. Larger publisher might not like steam so much because they have gigantic marketing budgets in the first place so all the discoverability/recommendation stuff is not as relevant but, well, fuck EA, Ubi, etc IDGAF, I don't want to hear it, cry silently.

As to the "can't sell games for lower prices elsewhere" myth: That applies to when you're selling steam keys in places that are not steam. Which is fair, if you're selling steam keys then that's incurring costs for them (if nothing else, bandwidth) and they don't even get their usual cut when you sell a steam key off-steam, least you can do is not undercut them.